1. Field of the Invention
This invention relates to a brake fluid pressure control apparatus for use in an anti-lock control for preventing the locking of vehicle wheels at the time of braking the vehicle and also to a brake fluid pressure control apparatus for use in both the above anti-lock control and a traction control for preventing a slip of the drive wheels at the time of starting the vehicle or accelerating it.
2. Prior Art
In an anti-lock control apparatus designed to prevent the locking of vehicle wheels at the time of braking the vehicle so as to ensure the steerability and running stability of the vehicle and to shorten the braking distance, the control modes of the brake fluid pressure (i.e., a pressure increasing mode, a pressure decreasing mode and a pressure holding mode) are determined in accordance with electrical signals from speed sensors for sensing the speed of the vehicle wheels. And, in accordance with the pressure control modes thus determined, hold valves and decay valves each in the form of a solenoid valve are opened and closed so as to increase, hold or reduce the brake fluid pressure. Such brake fluid pressure control is effected under the control of a microcomputer.
There is known a brake fluid pressure control apparatus of the type in which a fluid pressure control portion (modulator) for effecting the above-mentioned anti-lock control is incorporated in a master cylinder. In such an apparatus as disclosed in U.S. Pat. No. 4,641,895, the master cylinder comprises a primary piston and a secondary piston arranged in series, which is known as the tandem type, and brake fluid pressures in two brake lines of the brake device of the dual circuit type are controlled by the primary and secondary pistons, respectively. The above-mentioned hold valves are provided respectively in fluid passages connecting two fluid pressure chambers respectively to wheel cylinders, the fluid pressures in the two fluid pressure chambers being controlled by the primary and secondary pistons, respectively. The decay valves are provided in fluid passages connecting the wheel cylinders respectively to a reservoir.
With this construction of the above conventional apparatus, normally, the hold valves are open whereas the decay valves are closed, so that upon pressing-down of a brake pedal, the fluid pressures in the two fluid pressure chambers are supplied to the respective wheel cylinders through the respective hold valves to increase the fluid pressures in the wheel cylinders, thereby applying a braking force to the wheels. And, when the anti-lock control is started, the hold valves are closed to hold or maintain the brake fluid pressures. Further, when the decay valves are opened, with the hold valves kept closed, the brake fluids in the wheel cylinders are escaped to the reservoir through the decay valves, so that the fluid pressures in the wheel cylinders are reduced, thereby reducing the braking force. Also, in the pressure increasing mode of the anti-lock control, the hold valves are operated into the open condition, and the pistons are operated by the brake fluid of high pressure supplied from a fluid pressure source such as an accumulator to thereby increase the fluid pressures in the wheel cylinders.
However, if any malfunction should develop in the fluid pressure source system, e.g., a fluid pressure pump, during the anti-lock control, the braking force becomes inadequate. To compensate for this, the brake pedal has to be further pressed down, so that the brake pedal can not be still further pressed down adequately (that is, there is no sufficient room for further pressing-down).
Another difficulty is that during the normal braking operation in which the anti-lock control is not effected, when vapor lock occurs in the brake fluid line due to the overheating of the brake device, a sufficient braking force can not be achieved even if the brake pedal is fully pressed down.
Further, in the case where in addition to the above-mentioned anti-lock control, the above brake fluid pressure control apparatus is designed to perform a traction control, the brake fluid of high pressure of the fluid pressure source as in the anti-lock control acts on the pistons, the braking force becomes excessive. This not only imparts vibration to the vehicle body to provide an uncomfortable ride but also to apply an undesirable load to the drive system.